13 April 2018

3GWB, 3G ALE extensions for wideband operations (Harris WB ALE)

I was investigating some wideband HF (WBHF) scenarios and spotted unexpected 3G-HF FLSU bursts in some recordings: it's a unexpected presence because for circuit mode connections WBHF is not comprised in the types of traffic that will be delivered on the link (the available types of traffic are listed in STANAG-4538 Table 4.6.1-2). Such transmissions can be observed by monitoring the band around 9.2 MHz.Reading the Appendix-G to MIL 188-141D, I found an answer to my perplexities: most likely those intercepts are related to 3GWB: a set of "extensions" to 3G-HF linking for wideband. Indeed, quoting MIL 188-1414D §G.5.5.7, "It is possible to set up a narrowband link using 3G-HF FLSU and then to negotiate a wideband channel for traffic via a second handshake that uses 3GWB extensions to the FLSU protocol, the link shall be terminated with FLSU_term. The extensions for this 3GWB mode are not standardized here". Figure 1 provides a timing diagram of all the signalling required. Harris developed a similar system for wideband ALE termed "WB ALE" [1] [2].

That said, I am inclined to say that part of the heard WBHF transmissions are just examples of WB ALE/3GWB system tests. Figure 2 reports a such transmission recorded on 5365.0 KHz/USB. The link setup of standard 3G-HF is unaffected: link request and confirm control FLSU bursts remain unchanged. Harris say that the only modification to 3G linking is the addition of a new traffic type to support wideband data (as I supposed). After the link has been established, both radios simultaneously perform a spectrum sensing operation to determine the currently interference-free bandwidth at each end. Once spectrum sensing has determined the available bandwidth and offset, a second two-way handshake (WB handshake) is used to exchange the required information for the wideband transfer. Once the data transfer is complete, FLSU terminate bursts will tear down the link as is done in 3G-HF.

Fig. 2 - on-air 3G wideband point-to-point link

Fig. 3

By the way, the traffic waveform (Fig. 4) has a symbol rate of 14400Bd and uses the 18 KHz bandwidth.

Fig. 4

I
do not have informations about the format of the FLSU PDUs (Protocol data Units) employed in the WB ALE handshake (are
they still addressed?) and the modded BW5 waveforms. Unfortunately, the quality of recordings doesn't
allow accurate investigations as the ACF and the period framing: I can only state that WB ALE FLSU request and
confirm bursts last about 530ms and are modulated at 2400 symbols/sec
using PSK-8 modulation. Quoting Harris [1]:"The two-way WB handshake has been designed using burst waveforms very similar to BW5 of STANAG-4538 and allows the exchange of the following information:

- Coordinated decision on offset (i.e. frequency offset of available bandwidth in the up to 24 kHz allocation; offset value is quantized and is in the range of +/- 10500 Hz)- Coordinated decision on initial data rates.

Bandwidth and offset decisions will be based on either a primary or secondary usage of channel. In primary user mode, bandwidth and offset decisions can be made independently for each direction of transmission. In secondary user mode, bandwidth and offset will be the same in both directions."

Given the latest "D" release of MIL 188-110 (December 2017), it would be interesting to know if Harris has upgraded WB ALE to support up to 48 KHz bandwidth waveforms introduced by the Appendix D.